bufmgr: Make UnlockReleaseBuffer() more efficient

bufmgr: Make UnlockReleaseBuffer() more efficient

Now that the buffer content lock is implemented as part of BufferDesc.state,
releasing the lock and unpinning the buffer can be implemented as a single
atomic operation.

This improves workloads that have heavy contention on a small number of
buffers substantially, I e.g., see a ~20% improvement for pipelined readonly
pgbench on an older two socket machine.

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/5ubipyssiju5twkb7zgqwdr7q2vhpkpmuelxfpanetlk6ofnop@hvxb4g2amb2d